This invention relates in general to the field of shaft couplings and in particular to a device for preventing flailing of a drive shaft having a failed torque transmission coupling and/or loss of a coupling fastener.
Without limiting the scope of the invention, its background will be described with reference to drive shaft couplings in helicopters as an example.
Helicopter engines provide power to several major components such as the main rotor, the tail rotor and the blower. The engines drive transmissions, which transmit rotational power to the components through various drive shafts. To ease installation and maintenance and to accommodate installed and induced angles, drive shaft segments may be coupled with torque-transmitting flexible diaphragm or disk joints such as THOMAS(copyright) couplings, for example.
THOMAS(copyright) couplings, manufactured by REXNORD(copyright), are non-lubricated, metal flexing couplings, having non-wearing parts to transmit torque and accommodate shaft misalignment. The flexible element is a series of precision stamped discs with uniquely designed cross sections that flex without causing the metal-to-metal wear problems associated with lubricated couplings. The series of discs are engineered for infinite life if the coupling is operated within specified environmental and load conditions. This conservative design standard assures maximum reliability on the most critical drive systems.
The most reliable mechanical system, however, remains susceptible to failures that are a result of material fatigue, manufacturing defects and human factors. THOMAS(copyright) couplings have several failure modes. For example, a coupling fastener that is under-torqued may become unfastened by vibration from the aircraft. Losing a coupling fastener will cause shaft to rotate out of alignment and the eccentric rotation will cause a catastrophic coupling failure. An over-torqued coupling fastener may cause stress fractures in the coupling discs, which may result in the discs shearing from the fasteners. Again, this will cause the shaft to rotate out of alignment, which results in coupling failure.
After a coupling failure, the free end of the shaft flails about and causes significant damage. The shaft driving the tail rotor, for example, rotates at a speed of approximately six-thousand one-hundred revolutions per minute (RPM). The tail rotor shaft includes several drive shaft segments that are coupled using THOMAS(copyright) couplings. A coupling failure releases a free end of one of the segments that is flailing at about six-thousand one-hundred RPM. The free end of the shaft segment may then tear the tail section from the aircraft, which results in a loss of directional stability. A flailing shaft may also damage or destroy- other proximate aircraft systems. For example, the blower shaft, which is rotating at approximately ten-thousand RPM, may destroy the aircraft""s hydraulic system if the coupling fails.
Therefore, a need has arisen for a shaft coupling assembly that will not allow the free end of the shaft to flail if the coupling fails. A need has also arisen for such a coupling assembly that does not add significant cost, complexity or weight to the aircraft drive systems. Further, a need has arisen for such a shaft coupling assembly that may be fitted to existing drive shafts.
The present invention disclosed herein provides an anti-flail assembly for preventing an uncoupled end of a rotating shaft from flailing. The anti-flail assembly has a flange, which has a face and a perimeter. The flange is attachable to an end of a shaft. A shoulder extends from the perimeter of the flange generally perpendicularly to the face of the flange. The shoulder of the anti-flail cup surrounds a shaft coupling and retains the shaft ends if the coupling fails.
In one embodiment of the present invention, a method for preventing an end of a rotating shaft from flailing is provided. The method includes attaching an anti-flail cup to an end of a first shaft. A second shaft is coupled to the end of the first shaft such that the anti-flail cup extends from the end of the first shaft to the coupled end of the second shaft.
In another embodiment of the invention, an anti-flail system includes a first shaft coupled to a second shaft by a coupling. An anti-flail cup is attached to the first shaft. The anti-flail cup retains the ends of the first shaft and the second shaft if the coupling fails.